Control Engineering

Paul Grayson

Intuitive Steer-By-Wire
November 26, 2007

Drive-by-wire systems are rapidly replacing mechanical linkages in transportation system.  Considering reliability critical, AIM began looking for examples of position sensors suitable for our driverless AGV WENDY DARLING, a retired 1954 GMC M-215 Army supply truck that we are converting.  Along the way we found some remarkable details about Dean Kamen's Segway.

The Segway Personal Transporter i2, depends on LeanSteer, a highly reliable closed-loop, steer-by-wire technology built around Novotechnikís redundant SP2880 rotary sensor.   The Personal Transporter (PT) is an example of an “inverted pendulum” and by using gyroscopes, tilt sensors, and electric motors in a tightly integrated system is able to balance the rider on two wheels.  To steer, the rider simply leans in the direction she wants to travel.  The handlebar shaft acts as a lever exerting force at the base where an SP2800 position sensor continuously measures the handle bar shaft’s angle while outputting real-time data to the closed-loop Balance and LeanSteer system.  It is not necessary for the rider to consciously push the handlebar to the left or right, the position sensor is sensitive down to hundredths of a degree and recognizes the slightest change in angle as the rider instinctively leans in the direction of travel.  “If the first Segway felt intuitive - lean forward to go forward, lean back to stop and reverse, twist your wrist to turn, the new models respond as if they’re controlled by mere thought.”  -- Time Magazine

Their challenge was to find a reliable sensor to integrate into the real-time control system.  LeanSteer controls the speed of each wheel independently to turn the Personal Transporter.  “Given the critical role played by the position sensor in the LeanSteer system, it was essential to find a sensor manufacturer that we could partner with - a technological leader who would work with us to meet our exacting requirements.” -- Michael Gansler, Segway Product Development Engineer

They picked Novotechnik, a company that is a proven supplier of position sensors to BMW, Daimler, Chrysler, Volkswagen, Nissan, Siemens, and Bosch for both on-road and off-road vehicles.  They have proven technology, automotive experience, volume-manufacturing capability, and the engineering support needed in mobile applications.  The SP2800 Series sensor used in the system was designed and validated to provide high precision and long-term durability together with the high bandwidth sensing capability needed for real-time control.  The design solution called for electrical redundancy to provide the fail-safe dependability required in critical steering and other by-wire systems.  Segway and Novotechnik packaged two independent potentiometers in a 28 mm by 17 mm housing then integrated them into the closed-loop system.  The control system makes real-time comparisons between two outputs with the requirement that the two measurements agree.  This acts as a full-time diagnostic, checking the sensors 100-times a second to ensure that they are both working perfectly.  Any failure to agree brings the PT to a graceful, safe shutdown.

The SP2880 potentiometric sensor features dual-tracks, redundant elastomer-damped, multi-fingered precious metal wipers, and dual output to ensure uninterrupted operation while delivering the constant, absolute, proportional analog output signal required to steer while maintaining balance.  The SP2800 Series features an extended lifetime of more than 50 million movements, provides a resolution of 0.01^o with independent linearity of ± 0.3% and repeatability of <0.03^o.  Engineered to resist vibration, SP2800 Series sensors are built with premium fiber-filled, temperature-resistant, dust-proof plastic bodies sealed to IP65 rating for reliable operation at temperatures from -40^oC to +150^oC.  The sensor’s mounting flanges have elongated holes to simplify installation and allow easy mechanical adjustment.  A push-on mechanical coupling simplifies installing the sensor into the handlebar pivot assembly and minimizes hysteresis due to play.

The Segway Smart Motion (SSM) and LeanSteer control system also includes two digital signal processors, two dual-axis accelerometers for tilt angle measurement, and five MEMS/RAS micro-machined gyroscopic angular rate sensors.  The angle information supplied by the SP 2880 sensor is used by the control system to generate the amount of lateral acceleration needed to balance the rider.

Novotechnik also developed the 48-bit DyMoS bus ASIC for customers who needed both position and actual velocity information as well as a data transfer error Cyclical Redundancy Check capability.  Their high-precision contact, non-contact rotary and linear position sensors are being used in a wide range of applications in the automotive, robotics, machine, hydraulic, pneumatic, measurement, and medical device industries.  They even have produced sensors for the  hydraulic lateral-control cylinders on Siemensí 200 MPH ICE 3 trains to compensate for lateral acceleration (lean) as the train travels through curves at high speeds, Airbus A3 joysticks, electronic valve control, and feedback control for mobile hydraulic pumps.  Perhaps sometime soon they will be able to add AIM's retired Army supply truck to their list of users?

GO ROBOTS !

Paul F. Grayson - Chief Engineer
AMERICAN INDUSTRIAL MAGIC, LLC
Racing to build technology that saves soldier's lives.
390 4-Mile Rd. S.
Traverse City, MI 49686-8411
(231) 946-0187, (231) 883-4463 Cell
pgrayson@aimagic.org
http://aimagic.org
http://tech.groups.yahoo.com/group/robotcluboftraversecitymi/
http://www.controleng.com/index.asp?layout=blog&blog_id=1180000318

Posted by Paul Grayson on November 26, 2007 | Comments (0)